SML2108F datasheet(19/21 Pages) SUMMIT

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2053 2.2 11/07/00

SML2108

SUMMIT MICROELECTRONICS, Inc.

PRELIMINARY

APPLICATION EXAMPLE USING EXTERNAL TEM-

PERATURE INPUT

The EXT TEMP pin of the SML2108 allows the input of the

internal ADC to be driven from an external device, rather

than a mirrored version of the bias current. Figure 25

shows an example using a National Semiconductor

LM334 to deliver a current into the SML2108 that is

proportional to absolute temperature.

The scale and

offset features of the ADC input can be used to center this

current and maximize the full range of the look-up table.

For this application the current ISET coming out of the

LM334 and into the EXT TEMP pin of the SML2108 is

given by the following equation:

ISET = 227µV / oK / RSET

For example, using a value of 210

Ω for RSET yields a

current of 295µA at 0oC and 387µA at 85oC.

Next select scale and offset values of the ADC input that

will optimize the current range of the LM334. Nominal full-

scale input current of the ADC is 390.6µA (= 100mA/256).

By setting the input offset to ¾ scale (293µA) and the full-

scale range to ¼ scale (97.6µA), then the zero scale of the

internal ADC becomes 293µA, and the full-scale is

390.6µA. This represents a temperature range of approxi-

mately –2oC to 88oC using a 210

Ω resistor in the configu-

ration shown. (These settings correspond to configuration

Figure 25. Example of an External Temperature

Sensing Device

APPLICATIONS

–

2053 Fig25

RSET

EXT TEMP

SML2108

LM334

210

Ω ±1%

ISET

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